Organic PTC thermistor device

ABSTRACT

An organic PTC thermistor device includes an organic PTC thermistor element having first and second surfaces opposite to each other; first and second electrode layers deposited on the first and second surfaces, respectively, and having defined therein respective non-electrode regions which are displaced in position with respect to each other; and first and second terminal members elastically engaged respectively to a portion of the first electrode layer, which is aligned with the non-electrode region in the second electrode layer and to a portion of the second electrode layer which is aligned with the non-electrode region in the first electrode layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an organic PTC (positivetemperature coefficient) thermistor device and, more particularly, tothe support of a thermistor element within a casing.

2. Description of the Background Art

Various PTC thermistors have long been used as protective circuitelements for protecting circuit component parts from the overcurrent. Ofthese various PTC thermistors, organic PTC thermistors are well knownand comprise a thermistor element made from electroconductive particlessuch as, for example, those of carbon black or any other metal, mixedinto synthetic resin of a polyolefin system such as, for example,polyethylene.

According to the background art, the thermistor element in an organicPTC thermistor is encased in a fashion shown in FIG. 9 of theaccompanying drawings.

Referring to FIG. 9 for the purpose of discussion on the prior art, theorganic PTC thermistor element is generally identified by 10 and is inthe form of, for example, a disc having on its opposite surfacesrespective, thermally-deposited metallic foils. These metallic foilsserve as electrodes 11 having respective leads 12 connected thereto bymeans of solder deposits 13. The assembly is then encased with an outercoating 14 of synthetic resin with outer end portions of the leads 12exposed to the outside of the outer coating 14 for electric connectionwith external circuit elements.

In the prior art PTC thermistor device of the construction shown in FIG.9, it has been found that the thermistor element 10 tends to deteriorateso much as to result in a loss of stability under the influence of heatevolved during the soldering of the leads 12 to the electrodes 11 and/orthe formation of the outer coating 14.

In view of the foregoing, the Japanese Laid-open Utility ModelPublication No. 61-201, published in 1986, has proposed an organic PTCthermistor device free from thermal influences. According to thispublication, the PTC thermistor device comprises an organic PTCthermistor element having electrodes deposited on the respectiveopposite surfaces thereof, which element is retained in position withina casing by means of a pair of terminal members elastically clamping theelement from opposite directions while held in contact with theelectrodes.

However, the PTC thermistor device disclosed in the above-mentionedpublication has been found having a problem in that, when the thermistorelement is heated as a result of an overcurrent induced in the elementduring its operation, the element, which is made from organic materialas its principal component, tends to be softened to such an extent thatresilient forces exerted by the terminal members and centered on therespective points of contact with the associated electrodes may causethe element to deform at two locations, corresponding respectively tothe points of contact of the terminal members with the electrodes, inrespective directions towards each other. In the worst case it mayhappen, the thickness of the thermistor element may be reduced at aportion where it is elastically clamped by the terminal members,resulting in shortcircuiting between the opposite electrodes.

The above-discussed problem may be obviated if the resilient forcesapplied from the terminal members to the element through the associatedelectrodes to retain the element in position are reduced. However, thereduction of the resilient forces may permit the element to undergoarbitrary motion within the casing under the influence of vibrationsand/or impacts and also to exhibit an increased contact resistanceaccompanied by change in operating performance.

SUMMARY OF THE INVENTION

The present invention has been devised with a goal of substantiallyeliminating the above-discussed problems and, in preferred form, relatesto an improved organic PTC thermistor device which comprises an organicPTC thermistor element having first and second surfaces opposite to eachother; first and second electrode layers deposited on the first andsecond surfaces, respectively; said first and second electrode layershaving respective non-electrode regions defined therein; saidnon-electrode regions in the first and second electrode layers beingdisplaced in position with respect to each other; and first and secondterminal members elastically engaged respectively to a portion of thefirst electrode layer, which is aligned with the non-electrode region inthe second electrode layer and to a portion of the second electrodelayer which is aligned with the non-electrode region in the firstelectrode layer.

According to the present invention, the first terminal member iselastically engaged to that portion of the first electrode layer whichis aligned with the non-electrode region in the second electrode layer,whereas the second terminal member is elastically engaged to thatportion of the second electrode layer which is aligned with thenon-electrode region in the first electrode layer. The elasticengagement of the respective first and second terminal members to theassociated portions of the first and second electrode layers is not onlyfor the purpose of electrically connecting the first and second terminalmembers with the associated electrode layers, but also for the purposeof supporting the thermistor element.

Because of the unique support system employed in the present invention,even though the thermistor element is softened as a result ofself-heating such element will not be deformed substantially becauseresilient forces of the terminal members are distributed. Should thethermistor element be deformed, shortcircuiting will not occur because aportion of one of the first and second surfaces of the thermistorelement that is opposite to that portion of the other of the first andsecond surfaces in contact with the associated terminal member throughthe associated electrode layer is deprived of an electrode; that is,aligned with the non-electrode region in the electrode layer on suchother of the first and second surfaces. Therefore, in the PTC thermistoraccording to the present invention, the resilient force applied by eachterminal member to the associated surface of the thermistor elementthrough the associated electrode layer can be chosen to be a sufficientvalue and, accordingly, the thermistor element can be held securewithout being adversely affected by external vibrations and/or impacts.The contact resistance between each terminal member and the associatedelectrode layer can also be stabilized.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects and features of the present inventionwill become clear from the following description taken in conjunctionwith preferred embodiments thereof with reference to the accompanyingdrawings, in which:

FIG. 1 is an elevational view of an organic PTC thermistor device, witha lid removed, according to a preferred embodiment of the presentinvention;

FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1;

FIG. 3 is a perspective view of an organic PTC thermistor element usedin the device of FIG. 1;

FIG. 4 is an elevational view of a thermistor element according toanother preferred embodiment of the present invention;

FIG. 5 is a top plan view of the element shown in FIG. 4;

FIG. 6 is a view similar to FIG. 4, showing a thermistor elementaccording to a further preferred embodiment of the present invention;

FIGS. 7 and 8 are top plan views of the element of FIG. 6, showingopposite surfaces of the element, respectively; and

FIG. 9 is a longitudinal sectional view of a prior art organic PTCthermistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before the description of the present invention proceeds, it is to benoted that like parts are designated by like reference numeralsthroughout the accompanying drawings except for FIG. 9.

Referring first to FIGS. 1 to 3, an organic PTC thermistor deviceembodying the present invention comprises an organic PTC thermistorelement 1 of any known construction made from electroconductiveparticles such as, for example, those of carbon black or any othermetal, mixed into a synthetic resin of a polyolefin system such as, forexample, polyethylene. As so far illustrated, the organic PTC thermistorelement 1 is generally in the form of a rectangular plate and hasopposite first and second surfaces deposited with respective first andsecond electrode layers 2 and 3. These first and second electrode layers2 and 3 may be formed by thermally depositing electroaonductive foils,or printing electroconductive paint material, on the respective firstand second surfaces of the thermistor element 1. Each of the first andsecond electrode layers 2 and 3 has a non-electrode or deprived region2a or 3a which allows a corresponding portion of the associated surfaceof the thermistor element 1 to be left uncoated by any electrodematerial.

In the illustrated embodiment, the first and second electrode layers 2and 3 have the respective non-electrode regions 2a and 3a definedtherein at such locations that the uncoated portions of the first andsecond surfaces of the thermistor element 1 can be offset relative toeach other in a direction parallel to the longitudinal sense of thethermistor element 1.

The thermistor device also comprises first and second terminal members 5and 6 each having an inwardly-bent elastic tongue 5a or 6a and fixedalong a corresponding inner wall portion of a casing 7, the elastictongues 5a and 6a elastically clamping the thermistor element 1 throughthe adjacent electrode layers 2 and 3 inwardly from opposite directionsto hold and retain the thermistor element 1 in position at a centralportion within the casing 7. The terminal members 5 and 6 are sosupported by and so positioned in the casing 7 that the elastic tongue5a of the terminal member 5 can be held in engagement with a portion ofthe first electrode layer 2 on the first surface of the thermistorelement 1 which is aligned with the uncoated portion of the secondsurface for the thermistor element 1. In contrast, the elastic tongue 6aof the terminal member 6 can be held in engagement with a portion of thesecond electrode layer 3 on the second surface of the thermistor element1 which is aligned with the uncoated portion of the first surface of thethermistor element 1.

At the same time, with the thermistor element 1 positioned inside thecasing 7, the respective uncoated portions of the first and secondsurfaces of the thermistor element 1 are engaged to associatedprojections 7a and 7b, integrally formed with the casing so as toprotrude inwardly thereof, whereby the resilient forces applied from theelastic tongues 5a and 6a to the thermistor element 1 can be received bythe projections 7b and 7a, respectively, to secure the thermistorelement 1 in position within the casing 7.

Hereinafter, the present invention will be demonstrated by way of someexamples which are not intended to limit the scope of the presentinvention.

Nickel foils were applied to the first and second surfaces of theorganic PTC thermistor element 1 and the assembly was subsequentlypressed under 120 kg/cm² for 10 minutes at 190° C. to complete the firstand second electrode layers 2 and 3. The organic PTC thermistor element1 was then cut into some sample chips of 15 mm in length, 10 mm in widthand 1.0 mm in thickness, followed by removal of portions of the firstand second electrode layers 2 and 3 to form the non-electrode regions 2aand 3a.

For the purpose of comparison, chips prepared in the same manner asdescribed above, but without the first and second electrode layers 2 and3 being partially removed, that is, having the first and secondelectrode layers completely covering the first and second surfaces ofthe thermistor element, were prepared.

Both of the sample chips according to the present invention and thecomparison were elastically sandwiched between the associated elastictongues within the respective casings.

TEST I

When the resilient force applied from each elastic tongue to thethermistor element was chosen to be 500 g and when a direct currentvoltage of 30 volts was applied between the terminal members,examination of the thermistor elements according to the illustratedembodiment of the present invention and according to the comparison hasrevealed that, while no change was found in the thermistor elementaccording to the illustrated embodiment, the thermistor elementaccording to the comparison showed a reduction in thickness of thatportion of the thermistor element where the associated elastic tongue isresiliently engaged with a consequent reduction in distance between thefirst and second electrode layers. In the prior art device, since thepoints at which the elastic tongues are engaged to the thermistorelement from the opposite directions confront with and are aligned witheach other, the resilient forces tend to be excessively centered on theterminal members and, therefore, the thermistor element is susceptibleto deformation. On the other hand, in the device according to theillustrated embodiment, since the points at which the elastic tonguesare engaged to the thermistor element are displaced from each other withthe resilient forces distributed, the thermistor element is lesssusceptible to deformation.

When direct current voltage of 30 volts was continuously applied to thesamples according to the illustrated embodiment of the present inventionand also according to the comparison, it has been observed that theelectrodes in the samples according to the comparison wereshortcircuited and burned out after the passage of 200 hours subsequentto the application of the direct current voltage, whereas no change wasfound in the samples according to the illustrated embodiment of thepresent invention. It is pointed out that, in the device according tothe illustrated embodiment of the present invention, even though thethermistor element undergoes deformation at portions where the elastictongues are engaged, no shortcircuiting occur because that portion ofthe thermistor element which is opposite to that portion of the same towhich the associated elastic tongue is engaged is occupied by thenon-electrode or deprived region in the associated electrode layer.

TEST II

As discussed above, the device according to the comparison is such that,when the resilient force exerted by each terminal member is increased,the thermistor element is susceptible to deformation. Therefore, astabulated below, a drop test was conducted, with the resilient forcereduced in both of Comparisons 1 and 2, to examine any possible changein resistance. During the drop test, two samples for each Comparison 1and 2 were dropped from a height of 0.75 meters down onto a woodenplate, 30×30 cm in size, made of wood from a maple tree.

    ______________________________________                                               Resilient Force                                                                           Before Test                                                                             After Test                                       ______________________________________                                        Embodiment                                                                             500 g         0.120 ohm 0.123 ohm                                             500 g         0.153 ohm 0.150 ohm                                    Comp. 1  100 g         0.183 ohm 0.223 ohm                                             100 g         0.153 ohm 0.187 ohm                                    Comp. 2   50 g         0.220 ohm 0.198 ohm                                              50 g         0.201 ohm 0.258 ohm                                    ______________________________________                                    

As can be understood from the above table, the two samples in each ofComparisons 1 and 2 have shown a considerable change in resistancebefore and after the drop test. This is illustrative of the fact that,because of insufficient resilient force to hold the thermistor elementin position, the thermistor element has undergone arbitrary motion andalso a change in contact resistance. In contrast the two samplesaccording to the illustrated embodiment have shown little change inresistance before and after the drop test and, accordingly, thethermistor device according to the present invention is excellent inresistance to vibration and also to impact.

In the embodiment shown in FIGS. 4 and 5, the thermistor element 1 is inthe form of a disc having a pair of opposite projections 1a and 1bprotruding radially outwardly therefrom in respective directions awayfrom each other. The first electrode layer 2 is deposited on the firstsurface of the thermistor element 1 including a continued surface of oneof the radial projections, for example, the projection 1b, whereas thesecond electrode layer 3 is deposited on the second surface of thethermistor element 1 including a continued surface of the other of theradial projections, that is, the projection 1a. The first electrodelayer 2 has the non-electrode or deprived region 2a defined therein at alocation corresponding to the projection 1a while the second electrodelayer 2 has the non-electrode or deprived region 3a defined therein at alocation corresponding to the projection 1b. When the thermistor elementof the construction according to the embodiment of FIGS. 4 and 5 ismounted and supported within the casing, the resilient forces exerted bythe elastic tongues 5a and 6a (see FIGS. 1 and 2) are applied to aportion of the first electrode layer 2 overlaying the projection 1b anda portion of the second electrode layer 3 overlaying the projection 1aas shown by the arrows A and B, respectively, in FIG. 4.

In the embodiment shown in FIGS. 6 to 8, the thermistor element 1 showntherein is similar to that shown in FIGS. 4 and 5 except that no radialprojection is employed. The circular thermistor element 1 shown thereinhas the first electrode layer 2 which is circular in shape and has adiameter smaller than the diameter of the thermistor element 1, so as toleave a non-electrode or deprived region 2a corresponding to aperipheral portion of the thermistor element 1 as best shown in FIG. 7.The second surface of the thermistor element 2 is deposited with thesecond electrode layer 3 which is of a generally-ring shape having anouter diameter equal to the diameter of the thermistor element 1 andhaving a central portion deprived to provide the non-electrode region 3aas best shown in FIG. 8. When the thermistor element of the constructionaccording to the embodiment of FIGS. 6 to 8 is mounted and supportedwithin the casing, the resilient forces exerted by the elastic tongues5a and 6a (see FIGS. 1 and 2) are applied to respective points indicatedby C and D, respectively, in FIGS. 7 and 8.

From the foregoing description, it has now become clear that the presentinvention is effective to provide the organic PTC thermistor devicewhich can advantageously withstand not only vibrations and impacts, butalso any possible thermal influence which may be brought about duringthe soldering of the thermistor device to external circuit elements. Inaddition, any possible deterioration of the thermistor element whichwill take place when coated with the external coating can also beeliminated.

It is also clear that, since the points at which the terminal membersare brought into contact with the thermistor element to retain thelatter in position are displaced or offset from each other, there is nosubstantial possibility that the distance between the electrode layersmay be considerably reduced to such an extent as to result inshortcircuiting therebetween. This advantage makes it possible to useterminal members capable of exerting an increased resilient forcerequired to allow the thermistor element to withstand vibrations and/orimpacts.

Although the present invention has fully been described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims.

I claim:
 1. An organic-PCT thermistor device which comprises:anorganic-PCT thermistor element having first and second surfaces oppositeto each other; first and second electrode layers deposited on the firstand second surfaces, respectively; said first and second electrodelayers having respective non-electrode regions defined therein; saidnon-electrode regions in the first and second electrode layers beingdisplaced in position with respect to each other; and first and secondterminal members elastically engaged respectively to a portion of thefirst electrode layer that is aligned with the non-electrode region inthe second electrode layer, and to a portion of the second electrodelayer that is aligned with the non-electrode region in the firstelectrode layer.
 2. The device as claimed in claim 1, wherein thethermistor element is rectangular in shape and wherein the non-electroderegion in the first electrode layer is defined at a locationcorresponding to one of the opposite ends of the thermistor element andthe non-electrode region in the second layer is defined at a locationcorresponding to the other of the opposite ends of the thermistorelement.
 3. The device as claimed in claim 1, wherein the thermistorelement is circular in shape and wherein the non-electrode region in thefirst electrode layer is defined at a location corresponding to aperipheral portion of the thermistor element and the non-electroderegion in the second electrode layer is defined at a locationcorresponding to a central portion of the thermistor element.